THE NYMPHAI MYSIAI (or Mysian Nymphs) were NaiadNymphsof the springs of Askanios in eastern Mysia (the north-western region of Anatolia). They seized the boy Hylas when he was sent by Herakles and the Argonauts to fetch water from their springs.

The story of the Mysian Nymphs was closely related to that of the Hyades, nymphs of the rain-bringing constellation of the same name. These raised their brother Hyas up to the stars when he was slain by a wild boar whilst fetching water.

ENCYCLOPEDIA

MYSIAN NYMPHS. When the Argonauts landed on the coast of Mysia, Hylas went out to fetch water for Heracles; but when he came to a well, his beauty excited the love of the Naiads, who drew him down into the water, and he was never seen again. (Comp. Val. Flacc. iii. 545; Orph. Argon. 637, &c.; Theocrit. xiii. 45, &c.) Heracles himself endeavoured to trace him, and called out his name, but in vain; and the voice of Hylas was heard from the bottom of the well only like a faint echo, whence some say that he was actually metamorphosed into an echo. While Heracles was engaged in seeking his favourite, the Argonauts sailed away, leaving Heracles and his companion, Polyphemus, behind. He threatened to ravage the country of the Mysians unless they would find out where Hylas was, either dead or alive. (Apollon. Rhod, i. 1344.) Hence, says the poet, the inhabitants of Cios (Prusa) still continue to seek for Hylas: namely, the inhabitants of Prusa celebrated an annual festival to the divine youth Hylas, and on that occasion the people of the neighbourhood roamed over the mountains calling out the name of Hylas. It was undoubtedly this riotous ceremony that gave rise to the story about Hylas. (Theocrit. xiii. 72; Strab. p. 564.)

Source: Dictionary of Greek and Roman Biography and Mythology.

Pseudo-Apollodorus, Bibliotheca 1. 117 (trans. Aldrich) (Greek mythographer C2nd A.D.) :
"Hylas whom Herakles loved, while off on a mission to fetch water [for the Argonauts while in Mysia], was kidnapped by Nymphai because of his beauty."

Apollonius Rhodius, Argonautica 1. 1225 ff (trans. Rieu) (Greek epic C3rd B.C.) :
"Hylas had bone off by himself [from the ship of the Argonauts beached near the river Kios in Mysia] with a bronze ewer in search of some hallowed spring where he could draw some water for the evening meal . . . Hylas soon found a spring, which the people of the neighbourhood call Pegai. He reached it when the Nymphai were about to hold their dances - it was the custom of all those who haunt (hyleoroi) the beautiful headland [of Pegai in Mysia] to sing the praise of Artemis by night. The Nymphai of the mountains (orea) and mountain torrents (enauloi) were all posted some way off to patrol the woods; but one, a Nymphe of the water (ephyatie), was just emerging from the limpid water as Hylas drew near.And there, with the full moon shining from a clear sky, she saw him in all his radiant beauty and alluring grace. Her heart was flooded by desire; she had a struggle to regain her scattered wits. But Hylas now leant over to one side to dip his ewer in: and as soon as the water was gurgling loudly round the ringing bronze she threw her left arm round his neck in her eagerness to kiss his gentle lips. Then with her right hand she drew his elbow down and plunged him in midstream . . .
[The Sea-God Glaukos to the Argonauts:] ‘As for Hylas, who caused these two to go astray and so be left behind, a Nymphe has lost her heart to him and made him her husband.’"

Z37.2 HYLAS,
MYSIAN NAIADES

Antoninus Liberalis, Metamorphoses 26 (trans. Celoria) (Greek mythographer C2nd A.D.) :
"When they [the Argonauts] reached the narrows of the Black Sea and were sailing past the headland of Arganthone, the waves began to toss in a storm. They dropped anchors and let the ship ride . . . The boy Hylas went carrying a pail to the River Askanios to fetch water for the leaders. And when the Nymphai, who were the daughters of this River, saw and fell in love with him, they pulled him in, dragging him down into the spring.
After Hylas had disappeared, Herakles saw that he was not coming back to him and deserted the heroes, searching everywhere in the thickets, calling ‘Hylas’ again and again. The Nymphai, fearing that Herakles might discover that they had hidden the lad among them, changed him into a n echo which again and again echoed back the cries of Herakles. After all his unavailing efforts to find Hylas, he returned to the ship and sailed away with the heroes. He left Polyphemos on the spot to search and, if he could, find Hylas for him. But Polyphemos died before he could succeed. To this day local people make sacrifices to Hylas by the spring. The priest calls him by his name three times and an echo replies three times."

Strabo, Geography 12. 4. 3 (trans. Jones) (Greek geographer C1st B.C. to C1st A.D.) :
"Here [Kios (Cius) in Bithynia] is the scene of the myth of Hylas, one of the companions of Herakles who sailed with him on the Argo, and who, when he was going out to get water, was carried off by the Nymphai. And when Kios, who was also a companion of Herakles and with him on the voyage, returned from Kolkhis (Colchis), he stayed here and founded the city which was named after him. And still to this day a kind of festival is celebrated among the Prousians, a mountain ranging festival, in which they march in procession and call Hylas, as though making their exodus to the forests in quest of him."

Propertius, Elegies 1. 20 (trans. Goold) (Roman elegy C1st B.C.) :
"Just so did the [River] Ascanius prove cruel to the Argonauts. Your flame resembles Hylas, son of Theodamas, not inferior to his beauty, not unlike in name. Whether you sail on the River that flows through Umbria’s woods, or bathe your feet in Anio's waters, or pace the shores of the Gigante's (Giants') strand [i.e. the land of Phlegra], or anywhere a river's wandering waters welcome you, ward off from him the ever lustful hands of the Nymphae (the Ausonian [Italian] Adryades are no less amorous than their sisters) . . .
Such woes the ill-starred wanderer Hercules endured in a foreign land when he wept by the unrelenting Ascanius. For once on a time, they say, the Argo . . . brought its hull alongside the cliffs of Mysia. Here the band of heroes set foot upon the peaceful shore and covered the ground with a soft carpet of leaves. But the squire [Hylas] of the invincible prince [Herakles] ranged farther afield to seek the choice water of a sequestering spring.
Pursuing him [Hylas], two brothers, sons of Aquilo [Boreas the North Wind] (now Zetes overtakes him, now Calais overtakes), pressed with airborne feet to snatch kisses, retreating each in turn to plant kisses from below. But he at wing’s length mocks them as they hover and wards off with a bough their winged assault. At last they of Pandion's line, the sons of Orithyia, gave up, and Hylas went on, alas, went on to the Hamadryades [they were usually called Naiades in the story].
Here beneath the crest of Arganthus' mount lay the well of Pege, a watery haunt dear the Nymphae Thyniae; overheard from deserted trees hung dewy apples, owing naught to the hand of man, and round about in the water-meadow grew white lilies mingled with crimson poppies. Now in boyish delight plucking these with delicate nail, putting flowers before his appointed task, and now unwarily bending over the beauteous pool, he prolongs his truancy because of its charming reflections.
At length, with lowered hands he prepares to cup the water, leaning on his right shoulder to draw a full measure. When the Dryades, fired by his beauty, abandoned in wonder their accustomed dance and on his slipping pulled him nimbly through the yielding water, then by the snatching of his body did Hylas cause a loud sound. In answer Hercules from afar thrice called out ‘Hylas!,’ but to him from distant hills the breeze echoes naught save the name."

Valerius Flaccus, Argonautica 3. 521 ff (trans. Mozley) (Roman epic C1st A.D.) :
"Glancing at the pine-clad ridges of the hills to leftward she [Hera] sees a comely troop of huntress Nymphae, the pride of woods and waves. Light bows and green armlets have they all, and a shaft of myrtle-wood with tight-drawn strap; knew-high are their skirts, and the straying tresses float and fall gently rippling to the band that confines the hidden breasts. Earth herself re-echoes the beating of the sisters' feet, and sends up grasses beneath their tender steps. Of these Dryope, hearing the crash of Hercules' advance, as the quarry fled before his shafts, had gone forward to view the havoc of the grove, and was returning to her spring, brining back from Hercules an awe-struck face. Her Juno [Hera], down-gliding from the heavens and leaning against a dark pine tree, summons to her side, and grasping her hand thus speaks with coaxing words: ‘He whom scorning so many suitors I appointed, O Nympha, for thy wedlock - lo! The lad is here, come hither in the Haemonian barque, bright Hylas; he is wandering through thy glades and over thy hills . . . How fair a hope have the Nymphae of Achaea lost! With what complaining will Boebe’s [a lake in Thessaly] brood [of Naiades] hear that thou hast stolen him from them! How sad will be the [Naiad] daughter of [the River] yellow Lycormas!’
So saying she puts up a swift hart through the trackless brushwood [and lures Hylas in pursuit of the deer away from Herakles] . . . The stag leads him far onward to where a bright fountain gushes forth, and with light bound [the stag] springs clear over the pool. Thus is the lad’s hope baffled nor is he fain to struggle farther; and since sweat had bathed his limbs and labouring breast, he greedily sinks beside the pleasant stream. Even as the light that shifts and plays upon a lake . . . so doth he shed a gleam upon the waters; he heeds not the shadow of the Nympha or her hair or the sound of her as she rises to embrace him. Greedily casting her arms about him, as he calls, alack! Too late for help and utters the name of his mighty friend [Herakles], she draws him down; for her strength is aided by his falling weight."

Valerius Flaccus, Argonautica 4. 22 ff :
"Lo! In a vision [to Herakles] the boy [Hylas] rises from the water's level, clad in saffron weeds, the gift of the unkind Nympha, and standing by his dear head utters such words as these: ‘Why, father, dost thou waste time in vain lament? Mine now by fate's appointing is this glade, this home, wither at cruel Juno's [Hera's] behest the wanton Nympha has stolen me; now doth she win me power to consort with the streams of Jove [Zeus] and the heavenly deities, and shares with me her love and the honours of the fountain.’"